The present invention concerns a reactor core for a light water cooled reactor, and a fuel assembly and a control rod constituting the reactor core and, more in particular, it relates to a reactor core for a light water cooled reactor, a fuel assembly and a control rod intended for Pu multi-recycling at a breeding ratio of about 1.0 or slightly greater than 1.0 while keeping the economical and safety performance to the same level as in a BWR (Boiling water reactor) now under operation, that is, while minimizing change of in core structures and maintaining a negative void coefficient.
In a nuclear reactor, consumption of fissionable materials such as uranium-235 and plutonium-239 and conversion of fuel fertile materials such as uranium-238 and plutonium-240 into fissionable materials take place by nuclear reactions. A ratio between an amount of fissionable materials contained in fuels taken out of a reactor core and an amount of fissionable materials contained in fuels to be loaded in the reactor core is referred to as a breeding ratio, and the breeding ratio is about at 0.5 in existent light water cooled nuclear reactors. As a method of effectively utilizing uranium resources, it has been considered to increase the breeding ratio.
Japanese Patent Laid-Open Sho 55-10591 or Nuclear Technology, vol. 59, 212-227 pp, 1982 disclose that the breeding ratio can be improved by densely arranging fuel rods in a triangular lattice pattern to reduce a water-to-fuel volume ratio in a pressurized water type nuclear reactor. However, the breeding ratio is about 0.9 at the greatest and fissionable materials have to be supplemented for continuous operation without lowering the power. For further increasing the breeding ratio, it may be considered to make a fuel rod gap narrower to further decrease the water-to-fuel volume ratio, but it suffers from a limit, for an embodiment is difficult to attain, in view of manufacture of fuel assemblies and ensurance of thermal margin.
On the other hand, Japanese Patent Laid-Open Hei 1-227993 discloses a method of effectively reducing the water-to-fuel volume ratio by utilizing steam voids generated in a reactor core, which is a feature of a boiling water type nuclear reactor. However, it has been shown in the prior art to make the plutonium breeding ratio (a ratio between the amount of fissionable plutonium contained in fuels taken out of a reactor core and an amount of fissionable plutonium contained in fuels loaded in the reactor core; breeding ratio relative to fissionable plutonium) to about 1, but it is not shown to make the breeding ratio (a value is smaller by about 4 to 5% than plutonium breeding ratio in a case of enriching natural uranium with plutonium) to about 1, or 1 or more. If the plutonium breeding ratio is about 1, it is necessary to enrich natural uranium with plutonium for continuing operation without lowering the power and the uranium resources can not be used up thoroughly. In the present invention, the breeding ratio of about 1 means a value 0.98 or more.